apt complains about invalid filename extensions after upgrade to ubuntu 16.04

[Dominicus]

I upgraded from 14.04 to 16.04 a few days ago.

Since then, whenever I run apt commands, I get the following warning/error:


N: Ignoring file '50unattended-upgrades.ucf-old' in directory '/etc/apt/apt.conf.d/' as it has an invalid filename extension

What is the significance of this, and how to resolve?

Thanks!

[deadflowr]

See: https://ubuntuforums.org/showthread.php?t=2342917
for similar problem and answer.

[Dominicus]

there is exquisite humour in the linked answer
thank you